(CNN) -- Syrian opposition activists slammed the United Nations on Sunday for suspending its observer mission in the nation, saying it is "unjustifiable and unacceptable" for the international community to fail to protect civilians from attacks.

The United Nations said Saturday it is pulling back its unarmed monitors because escalating violence is limiting their ability to observe and verify reports, a major blow to a proposed peace plan.

"The decision of the U.N. supervision mission in Syria to suspend the monitoring mission represents a failure of ... the international community to effectively and responsibly deal with the situation in Syria," the opposition Local Coordination Committees of Syria said in a statement.

At a meeting of the Syrian National Council in Istanbul, officials from the opposition organization said the freezing of the observers' work signaled that it was time for world leaders to take more severe steps.

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"This mission, unfortunately, has not succeeded. This very small and minimum protection that we were hoping to find with the observers is not there anymore," said Basma Kadmani of the SNC.

SNC officials called on the U.N. Security Council to consider a resolution allowing the use of force in Syria.

"We say that all options are there and must be put on the table," said Abdul Basit Sieda, the group's leader. "This regime only understands the language of violence and force."

The peace plan brokered by Kofi Annan, international special envoy to Syria, had become a symbol of hope for civilians under constant attacks during the 15-month uprising. Annan's plan included a provision for the warring sides to lay down their weapons.

Violence has escalated in recent days, exacerbating an already risky situation for the about 300 monitors, said Gen. Robert Mood, who heads the U.N. supervision mission in Syria.

"Civilians continue to be trapped by the escalating violence in Syria," Mood said in a statement Sunday. "In Homs, attempts to extract civilians from the line of fire over the past week have been unsuccessful."

He called on government and opposition forces to allow women, children, the elderly and injured victims to leave conflict zones, saying the U.N. mission would monitor their release.

At least 51 people were killed nationwide Sunday, including 15 in Homs, opposition activists said, blaming government forces for the violence.

"The food supplies are very scarce and medical supplies are not there anymore, and...there are wounded people that need to be gotten out of the region, but this criminal regime is still trying to (commit) atrocities," Sieda said.

The rising violence has forced observers to stop conducting patrols and stay in their locations until it is fit for them to resume their mandated activities, Mood said.

Opposition members decried the timing of the mission's suspension and accused the international community of not having a game plan to help Syria, saying that only emboldens the regime.

"In the absence of any vision to push for an improvement in the situation, the current decision allows for more bloodshed and enables the regime to buy more time under international cover," the LCC said.

Syria blamed the uptick in violence -- and the decision to suspend the U.N. mission -- on "armed terrorist groups," a stance it has held consistently.

Syrian opposition groups estimate that more than 12,000 and 14,000 people have died since President Bashar al-Assad's government started cracking down on anti-government protesters last year.

There was a slow-down in killings when the cease-fire went into effect on April 12. But the pace picked up again, and the violence has soared in recent days with reports of heavy government bombardment on cities and chilling massacres of civilians.

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U.N. monitors witnessed the aftermath of some recent violence. For example, some entered the town of Houla a day after opposition activists reported a slaughter there. They also arrived in Qubeir after an alleged massacre there, though they were late after being turned back at checkpoints.

A few days ago, U.N. teams were turned back as they tried to reach al Haffa, which had been under bombardment for eight days. By the time the monitors arrived, the town lay charred and deserted, with an overwhelming stench of death.

On some occasions, the monitors have come under fire.

In the United States, the Obama administration has renewed calls for the regime to uphold its commitment to the peace plan.

U.S. officials are working with allies "regarding next steps toward a Syrian-led political transition" without al-Assad, according to Tommy Vietor, a spokesman for the National Security Council.

"The sooner this transition takes place, the greater the chance of averting a lengthy and bloody sectarian civil war," he said.

Meanwhile, Syria state media showed a meeting of the nation's newly-elected members of parliament.

The meeting Sunday discussed the formation of committees mandated by al-Assad to address public freedoms, human rights, women's rights and free press, according to state television.

A new parliament was voted in last month in elections boycotted by opposition groups, according to state media.